The invention provides a mercury vapor continuous monitoring device and monitoring method based on diode laser, belonging to the technical field of mercury vapor monitoring. The invention enables the problems that a conventional mercury vapor measuring system has a complex structure and real-time monitoring of mercury emission in conventional mercury vapor measuring is poor to be overcome. The monitoring device comprises a signal generator, a first laser diode controller, a second laser diode controller, a first laser diode, a second laser diode, a first reflector, a dichroic beam combiner, afirst convex lens, BBO crystals, a second convex lens, a beam splitter prism, a second reflector, a spectroscope, a sample cell, a reference cell, a first optical filter, a second optical filter, a first detector, a second detector and a data acquisition analyzer. According to the monitoring method, diode laser absorption spectroscopy is used to realize continuous monitoring of the concentration of mercury vapor, and spectral information of reference gas is utilized to realize selective identification and quantitative detection of gaseous elemental mercury, thereby eliminating interference brought by gas like sulfur dioxide and nitrogen dioxide. The monitoring device and the monitoring method provided in the invention are used for on-line monitoring of mercury vapor.